Ground-based Fabry-Perot observations of the hydrogen Balmer-alpha
emission have been used since the late 1970s to investigate hydrogen in
the geocorona, spanning the upper thermosphere and exosphere. Atomic
hydrogen in this region is a byproduct of hydrogen-containing species
below such as methane and water vapor. Models have predicted 50-75 %
increases in upper atmospheric hydrogen as a consequence of a doubling
of tropospheric concentrations of methane, a primary greenhouse gas. The
11-year solar cycle is a dominant source of natural variability in the
upper atmosphere and its effect on hydrogen distributions and emissions
must be understood to investigate possible signs of longer-term climatic
trends. We will discuss data from the present near-solar minimum winter
compared with those from the previous near- solar minimum period of
1997, all taken with the same instrument, the Wisconsin H-alpha Mapper
Fabry-Perot (Kitt Peak, AZ), and using the same nebular calibration
source for absolute intensity calibration. The newer data are consistent
with observations over the rise of the solar cycle with lower
intensities observed during solar minimum compared with solar maximum
conditions. We will also discuss preliminary comparisons with earlier
data and the extra challenges associated with comparing data taken with
different, though similarly designed instruments. The geocoronal
hydrogen column emission observed by the Fabry-Perot is a function of
the hydrogen density profile, the solar excitation flux, and radiative
transfer including the contribution of multiple scattering below the
Earth's shadow. We will discuss work in progress to use forward
modeling to retrieve the hydrogen column abundance from the emission
observations.